Flavonoids antioxidant function

The inhibition of lipid (LH) oxidation may be considered as one of the most important chemical reaction mechanisms that could explain the antioxidant function of flavonoids. In general terms, chain-breaking antioxidants (AH) inhibit or retard lipid oxidation (reactions 1-7) by interfering with initiation [generically represented by reaction 1] or with chain propagating reactions (reactions 2 and 3) by readily donating hydrogen atoms to lipid peroxyl radicals (LOO ) or lipid radicals (L ) (reactions 4 and 5) [Frankel, 1998] ... 

Beutner, S., Bloedom, B., Frixel, S., Blanco, L, Hoffmann, T., Martin, H., Mayer, B., Noack, P, Ruck, C., Schmidt, M., Schulke, L, Sell, S., Ernst, H., Haremza, S., Seybold, G., Sies, H., Stahl, W., and Walsh, R. 2001. Quantitative assessment of antioxidant properties of natural colorants and phytochemicals carotenoids, flavonoids, phenols and indigoids. The role of P- carotene in antioxidants functions, J. Sci. Food Agric., 81, 559. 

Beutner, S. et al. Quantitative assessment of antioxidant properties of natural colorants and phytochemicals carotenoids, flavonoids, phenols and indigoids the role of P-carotene in antioxidant functions, J. Set Food Agric., 81, 559,2001. 

Luximon-Ramma A, Bahorun T, Crozier A, Zbarsky V, Datia KP, Dexter DT, Aruoma OI (2005) Characterization of the antioxidant functions of flavonoids and proanthocyanidins in Mauritian black teas. Food Res Int 38 357-367... 

In plants, phenolic metabolites can stimulate cellular protective response coupled to antioxidant function in the presence of biotic and abiotic stress (Briskin 2000). Among abiotic stress, UV light induces phenolic phytochemicals through the phenylpropanoid and flavonoid glycoside pathways as a protective of metabolic response (Logemann et al. 2000). This UV inducible phenolic phytochemical response can help to counter intracellular ROS produced in response to UV. This UV-inducible phenolic response ean be coupled to antioxidant enzyme response (Rao 1996) to attenuate damage from UV radiation. 

The antioxidant properties of flavonoids have been recognized since the middle of the last century. Quercetin is probably the active compound that gathers all the properties necessary for a powerful antioxidant function, and it is used in moisturizing and anti-aging products. 

There are numerous synthetic and natural compounds called antioxidants which regulate or block oxidative reactions by quenching free radicals or by preventing free-radical formation. Vitamins A, C, and E and the mineral selenium are common antioxidants occurring naturally in foods (104,105). A broad range of flavonoid or phenoHc compounds have been found to be functional antioxidants in numerous test systems (106—108). The antioxidant properties of tea flavonoids have been characterized using models of chemical and biological oxidation reactions. 

It is possible that dietary flavonoids participate in the regulation of cellular function independent of their antioxidant properties. Other non-antioxidant direct effects reported include inhibition of prooxidant enzymes (xanthine oxidase, NAD(P)H oxidase, lipoxygenases), induction of antioxidant enzymes (superoxide dismutase, gluthathione peroxidase, glutathione S-transferase), and inhibition of redox-sensitive transcription factors. 

Phytochemicals or phytonutrients are bioactive substances that can be found in foods derived from plants and are not essential for life the human body is not able to produce them. Recently, some of their characteristics, mainly their antioxidant capacity, have given rise to research related to their protective properties on health and the mechanisms of action involved. Flavonoids are a diverse group of phenolic phytochemicals (Fig. 6.1) that are natural pigments. One function of flavonoids is to protect plants from oxidative stress, such as ultraviolet rays, environmental pollution, and chemical substances. Other relevant biological roles of these pigments are discussed in other chapters of this book. 

Freedman and others (2001) determined the effects of purple grape juice and its main flavonoids on the functionality of platelets and the production of NO. They observed that incubation of platelets with diluted grape juice resulted in the inhibition of aggregation, increased production of NO, and decreased production of superoxide. To confirm the relevance of these findings, 20 healthy subjects were supplemented with 7 mL of black grape juice/kg/day for 14 days. The inhibition of platelet aggregation was also observed ex vivo there was an increase in the production of NO from 3.5 1.2 to 6.0 1.5 pmol/108 platelets and a decrease in the release of superoxide, from 29.5 5.0 to 19.2 3.1 arbitrary units. Under these conditions the antioxidant capacity of protein-free plasma increased by 50% (Freedman and others 2001). 

As has been explained in previous chapters, the antioxidant capacity of fruits and vegetables is a function of the amounts and types of phytochemicals that are present in the fresh tissues. However, the individual contribution to the total antioxidant capacity varies widely. Various studies have demonstrated that phenols and flavonoids contribute to a higher extent than ascorbic acid, carotenoids, and others to the antioxidant capacity of fmits and vegetables (Robles-Sanchez and others 2007). It has been observed that a given content of vitamin E in fruits contributes significantly more to the antioxidant capacity than the same content of ascorbic acid. 

Huge literature on biological functions of flavonoids and their antioxidant and free radical scavenging activities successfully competes with work on antioxidant effects of vitamins E and C. Flavonoids have been reported to exert multiple biological effects and exhibit antiinflammatory, antiallergic, antiviral, and anticancer activities [85 89], However, considering flavonoids as the inhibitors of free radical-mediated processes, two types of their reactions should be discussed flavonoids as free radical scavengers (antioxidants) and flavonoids as metal chelators. 

In addition to the induction of anthocyanin biosynthesis, chilling stress has also been shown to promote the formation of colorless flavonoids. Cold treatments (and drought stress) caused increases in levels of (—)-epicatechin and hyperoside (quercetin 3-galactoside) in two species of hawthorn, Crataegus laevigata and C. monogyna. Such treatments also enhanced the antioxidant capacity of the shoot extracts, and this may be the primary function of these cold-inducible flavonoids. 

Flavonoids often participate in plant reproduction, in the protection of reproductive tissues and seeds, and in seedling development. This may, in part, be due to their role in UV light shielding (thereby protecting DNA) and antioxidant properties, but other functions are also important. 

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